This invention relates to a coated type ink jet recording sheet. More particularly, it relates to a coated type ink jet recording sheet for use in an ink jet recording system that employs water-base ink, the sheet exhibiting a high rate of absorption of the ink deposited on the recording sheet so as to allow a clear color to develop, providing an image having circular dots and sharply defined edges, having a high surface strength and the same touch and feel during use as plain paper, and being able to record images with high resolution that are water-proof.
The ink jet recording sheet of the present invention may be used in recording, printing and writing employing water-base ink.
In order for an ink jet recording system to be able to produce high quality recorded images, it is required that the substrate employed be such that the ink deposited on the surface of the recording sheet which is composed mainly of paper spreads to form a true circle and is absorbed quickly in a controlled manner without becoming irregularly spread on the surface, and that the surface structure be such as to promote coloration of the deposited ink. Various proposals have so far put forward with a view to realizing these requirements.
For example, Japanese Patent Publication (KOKOKU) No. 27588/1985 discloses an ink jet recording paper wherein coating layer is applied with not less than 1 to 5 g/cm.sup.2 of a binder to one surface of a substrate paper having a Stockigt sizing degree of not more than 3 seconds such that ink droplets deposited on the paper surface are quickly absorbed within the interior of the sheet and spreading of the ink dots on the paper surface is prevented.
However, when the Stockigt sizing degree of the substrate sheet is lowered for this purpose, the water-proof quality of the deposited ink will be inadequate and such phenomena as strike-through, irregular ink spreading and inferior coloring will occur. In order that the capacity to absorb ink may be improved to provide a higher ink absorption rate and that spreading of the ink may be controlled so as to realize a dot shape close to a true circle without adversely affecting the water-proof quality and the color brightness, it is necessary to drastically increase the amount of coating material coated on the substrate surface to a value not less than 10 g/m.sup.2. With a coating amount of not more than 10 g/m.sup.2, the dot shape and coloration are not satisfactory and the dot size control that is necessary for good quality image reproduction cannot be achieved. However, if the coating amount is increased, the cost is raised and the feel of the plain paper and its suitability for use and converting.
Non-coated type ink jet paper has recently become popular in view of its suitability from the standpoint of cost, printing quality and adaptability to use as office paper. It is known in regard to the production of such ink jet paper for liquid-absorbing pigments such as silica or heavy calcium carbonate to be added to the raw materials at the time of making the paper. However, this type of ink jet paper presents certain problems in that the ink may not develop sufficient color brightness when applied to and absorbed by the paper on account of certain optical properties of the pulp and filler customarily used so that clear coloration of a quality comparable to that obtained when using coated paper is not attainable. Furthermore spreading of the ink cannot be prevented and the ink absorption rate and capacity are low.
For applications in which importance is attached to the brightness of color images, a coated type of paper is employed which has a coating layer consisting essentially of white pigments having superior water absorption properties so that the above-described dot shape and color brightness can be obtained.
In the absence of a specialized type of processing, it is impossible to provide water-proof images if water-base ink is used with any of these types of paper. For this reason, it is necessary to perform a water-proofing operation using a cationic surfactant or cationic polymer.
However, the conventional water-proofing agents used for ink jet images, such as polyethyleneimine and copolymers thereof (see, for example, Japanese Patent Publication Nos. 120486/1982 and 129778/1982), have only poor water-proofing ability and tend to lower the weatherability qualities of the images formed. The provision of a coating of a water-soluble polymer is disclosed in Japanese Patent Publication (KOKAI) No. 8685/1983, while Japanese Patent Publication (KOKAI) No. 55283/1983 discloses use of polyvinylpyrrolidone and hydroxyethyl cellulose. However, it has been pointed out that the dot shape attainable in these cases is poor and the surface strength inadequate, the weatherability properties also being adversely affected. Japanese Patent Application (KOKAI) Nos. 20696/1984, 109894/1985 and 74880/1986 discloses the use of water-soluble metal salts or cationic polymers, such as polydiallyl dimethyl ammonium chloride. This approach is not satisfactory either, however, in that the degree of water-proofing is insufficient and the weatherability properties and, above all, the color fastness, are adversely affected.
It is thus an object of the present invention to provide a coated type sheet for ink jet recording in an ink jet recording system employing a water-base ink which not only allows the water-base ink to be absorbed quickly within the surface region of the recording sheet, but also provides for good ink image color brightness, the formation of ink dots which are truely circular in shape and the diffusion of which is inhibited, and the formation of sharp-tone images and allowing the production of high-resolution images at high speeds, thereby overcoming the deficiencies of the coated or filled type sheets of the prior art.
It is another object of the present invention to overcome the deficiencies of the water-proofing agent used in the aforementioned prior art and to provide a coated type ink jet recording sheet that offers superior image quality and water-proofness.
The recording sheet of the present invention may be used in the same way as other types of recording sheet with water-base ink employing pen plotters or the like.